Silicon nitride is of great interest for structural and electronic applications because of its excellent high temperature strength, good thermal shock resistance, good wear resistance and chemical inertness. Silicon nitride materials have traditionally been fabricated by one of three methods: (1) the direct nitridation of silicon powder at high temperatures, (2) densification of commercially available silicon nitride powder at high temperatures by the addition of metal oxide powders known as sintering aids, and (3) densification of commercially available silicon nitride powder at high temperatures and pressures, a process known as hot pressing. These silicon nitride materials have been used in structural applications such as cutting tools and gas turbines.
Although silicon nitride ceramics having satisfactory mechanical properties at ambient and modestly high temperatures are available, many are not suitable for high temperature applications because their toughness, wear resistance and strength are often degraded at temperatures greater than 1000.degree. C. In the case of cutting tool inserts, wear resistance is a particularly important property. Undesirable phase transformations within the insert and chemical reactivity with the metal being worked on cause a deterioration in properties as the temperature rises at the high feeding and rotating speeds used for milling and turning operations.
Various methods have been proposed for improving the high temperature properties of silicon nitride ceramics. For example, a sintered ceramic product exhibiting high strength, toughness and hardness at elevated temperatures is described in U.S. Pat. No. 5,023,214. The product consists of silicides of Fe, Ni or Co uniformly dispersed in a silicon nitride matrix. Silicon nitride compositions containing MoSi.sub.2 have also been prepared. For example, U.S. Pat. No. 3,002,936 discloses a sintered mixture of silicon nitride and MoSi.sub.2 for applications requiring low electrical resistance as well as high thermal shock resistance. U.S. Pat. No. 3,409,417 discloses a dense, homogeneous interdisposition of silicon nitride and molybdenum metal in which up to half the volume of silicon nitride can be replaced by the silicides and disilicides of Cr, Mo and W. U.S. Pat. No. 4,407,971 describes a sintered ceramic body having high density, high flexural strength, and high impact resistance comprising silicon nitride, MoSi.sub.2, AlN, yttria and alumina. U.S. Pat. No. 4,644,133 discloses a ceramic heater element made from a mixture including MoSi.sub.2 and silicon nitride powder as a main ingredient and silicon dioxide powder as an additive. U.S. Pat. No. 4,983,554 discloses a sintered silicon nitride ceramic consisting of silicon nitride, MoSi.sub.2, AlN, yttria and alumina. The sintered compositions are formed by pressing a mixture of the powders at room temperature, followed by firing in a nitrogen atmosphere at 1700.degree. to 1800.degree. C. However, no compositions have been disclosed that comprise a homogeneous dispersion of a Mo.sub.5 Si.sub.3 phase in a silicon nitride matrix.